This invention relates generally to in situ leaching of mineral values, particularly uranium values, from subterranean formations. More specifically, this invention relates to processes for the recovery of uranium from formations associated with high brine aquifers. Improved yields and leaching rates are obtained while minimizing deleterious environmental impact such as groundwater and air pollution.
Uranium formations in high brine aquifers have been discovered in South Texas. These formations present special obstacles to leaching the uranium in situ because of the high brine content of the aquifers, such waters generally being saturated in CaCO.sub.3 and containing very high levels of NaCl. A typical high brine content aquifer, for example the West Ranch water, contains 60 g/l NaCl, 3 g/l CaCl.sub.2, 1 g/l MgCl.sub.2 and 0.5 g/l NaHCO.sub.3.
These formations present serious problems for uranium leaching. Thus, the high carbonate content of the aquifers makes it economically impractical to employ a conventional acid leaching technique. On the other hand, the high calcium levels in such aquifers make it impossible to leach with an alkaline carbonate solution without plugging the formation.
In addition, the high NaCl content in such aquifers presents different, but more significant, problems. A marked decrease in the leaching rate itself is experienced. For example, in waters with a NaCl concentration of 60 g/l, the leaching rate is slower than the leaching rate in fresh water by a factor of 5. Moreover, chloride ion interferes with the recovery of uranium by ion exchange processes.
As an alternative to acid leaching, CO.sub.2 /O.sub.2 -water systems have been used as lixiviants for leaching uranium formations. CO.sub.2 /O.sub.2 -water lixiviants are preferred to the ammonium carbonate/bicarbonate systems since the latter create the possible threat of ground water contamination through ammonium ion exchange with calcium and sodium ions contained in clays in the subterranean formation. The overall reaction using the CO.sub.2 /O.sub.2 lixiviant, i.e. oxidation of uranium to the hexavalent state and solubilization thereof, is shown by the following equation: EQU UO.sub.2 (S)+[O]+3HCO.sub.3.sup.- .fwdarw.UO.sub.2 (CO.sub.3).sub.3.sup.-4 +H.sup.+ +H.sub.2 O